Optical semiconductor device and method of controlling the same

ABSTRACT

An optical semiconductor device includes a waveguide and a refractive index control portion. The waveguide has one or more first segments, one or more second segments and a plurality of third segments. The first segment has a region that includes a diffractive grating and another region that is a space region combined to the region. The second segment has a region that includes a diffractive grating and another region that is a space region combined to the region. A length of the second segment is different from that of the first segment. The third segment has a region that includes a diffractive grating and another region that is a space region combined to the region. A length of the third segment is shown as L 3 =L 1 +(L 2 −L 1 )×K 1  in which 0.3≦K 1 ≦0.7, L 1  is a length of the first segment, L 2  is a length of the second segment and L 3  is a length of the third segment. The refractive index control portion controls refractive index of the first segment through the third segments.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates an optical semiconductor device and amethod of controlling the same.

2. Description of the Related Art

A wavelength-tunable semiconductor laser lasing some wavelength with asingle element is being developed in many organizations, along with aprevalence of Wavelength Division Multiplexing (WDM) communication usingan optical fiber. A wavelength-tunable semiconductor laser proposedbefore now is roughly classified into two types. One of the two typeshas a Semiconductor Optical Amplifier (SOA) in an external resonator,and controls a lasing wavelength with use of a wavelength selectableportion included in the external resonator. The other of the two typeshas a structure in which a wavelength-selectable resonator is built in asemiconductor element having a gain with respect to the lasing.

A representative wavelength-tunable semiconductor laser, in which theresonator is built in the semiconductor element, is a laser having aSampled Grating Distributed Reflector (SG-DR) waveguide. U.S. Pat. No.6,590,924 (hereinafter referred to as Document 1) discloses a laserusing a vernier effect. In the laser, a first SG-DR waveguides iscombined to a first side of a waveguide having a gain for lasing, and asecond SG-DR waveguide having a different longitudinal mode intervalfrom the first SG-DR waveguide is combined to a second side of thewaveguide. If a reflection peak wavelength of the first SG-DR waveguideand a reflection peak wavelength of a longitudinal mode of the secondSG-DR waveguide are changed according to temperature and current, thelaser oscillates at a wavelength where each of the peak wavelengthcorresponds to each other.

However, the laser disclosed in Document 1 may oscillate at a wavelengthother than a desirable wavelength because of regression wavelengthcaused by the vernier effect.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand provides an optical semiconductor device and a method of controllingthe optical semiconductor device that restrain a lasing at a wavelengthother than a desirable wavelength.

According to an aspect of the present invention, there is provided anoptical semiconductor device including a waveguide and a refractiveindex control portion. The waveguide has one or more first segments, oneor more second segments and a plurality of third segments. The firstsegment has a region that includes a diffractive grating and anotherregion that is a space region combined to the region. The second segmenthas a region that includes a diffractive grating and another region thatis a space region combined to the region. A length of the second segmentis different from that of the first segment. The third segment has aregion that includes a diffractive grating and another region that is aspace region combined to the region. A length of the third segment isshown as L3=L1+(L2−L1)×K1 in which 0.3≦K1≦0.7, L1 is a length of thefirst segment, L2 is a length of the second segment and L3 is a lengthof the third segment. The refractive index control portion controlsrefractive index of the first segment through the third segments.

With the structure, there is a wavelength dependence of peak reflectionintensity, because there are the segments having different length fromeach other. That is, the peak reflection intensity is relatively high ina given wavelength range. It is possible to restrain a laser oscillationat a wavelength other than a desirable wavelength when a lasingwavelength is one of wavelengths in the wavelength range where the peakreflection intensity is relatively high.

According to another aspect of the present invention, there is provideda method of controlling an optical semiconductor device, comprisingcontrolling refractive index of a first segment through a third segmentby controlling a refractive index control portion with use of afollowing expression shown as n3=n1+(n2−n1)×K2 in which 0.3≦K2≦0.7, n1is an equivalent refractive index of the first segment, n2 is anequivalent refractive index of the second segment and n3 is anequivalent refractive index of the third segment. The opticalsemiconductor device has the refractive index control portion and awaveguide including the first through the third segment. The firstsegment has a region that includes a diffractive grating and anotherregion that is a space region combined to the region. The second segmenthas a region that includes a diffractive grating and another region thatis a space region combined to the region. A length of the second segmentis different from that of the first segment. The third segment has aregion that includes a diffractive grating and another region that is aspace region combined to the region. A length of the third segment isshown as L3=L1+(L2−L1)×K1 in which 0.3≦K1≦0.7, L1 is a length of thefirst segment, L2 is a length of the second segment and L3 is a lengthof the third segment. The refractive index control portion controls therefractive index of the first through the third segments.

With the method, peaks in reflection peak intensity of an opticalwaveguide formed with the first segment and the second segment arealternately reduced because of antiphase peaks of reflection peakintensity of the third segment and the first segment and antiphase peaksof reflection peak intensity of the third segment and the secondsegment. That is, the intensity of the reflection peaks having desirablereflection intensity is alternately reduced. Further, the intensity ofthe antiphase peaks is increased because there are a plurality of thethird segments. This results in increase of the wavelength interval ofthe reflection peak having the desirable reflection peak intensity. Itis therefore possible to restrain the laser oscillation at a wavelengthother than the desirable wavelength.

According to another aspect of the present invention, there is provideda method of controlling an optical semiconductor device comprising:getting two of control points according to each of a first segmentthrough a third segment; calculating a lacking control point with use ofa following expression shown as T3=T1+(T2−T1)×K3 in which 0.3≦K3≦0.7, T1is a control point of the first segment, T2 is a control point of thesecond segment and T3 is a control point of the third segment; andcontrolling each refractive index of the first segment through the thirdsegment with the control points according to each of the segment. Theoptical semiconductor device has a waveguide including the first throughthe third segment. The first segment has a region that includes adiffractive grating and another region that is a space region combinedto the region. The second segment has a region that includes adiffractive grating and another region that is a space region combinedto the region. A length of the second segment is different from that ofthe first segment. The third segment has a region that includes adiffractive grating and another region that is a space region combinedto the region. A length of the third segment is shown asL3=L1+(L2−L1)×K1 in which 0.3≦K1≦0.7, L1 is a length of the firstsegment, L2 is a length of the second segment and L3 is a length of thethird segment.

With the method, the number of the control data may be reduced. In thiscase, the generation of the control data may be simplified. And hardwareresource during the use of the optical semiconductor device may bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a laser device in accordance with a first embodiment;

FIG. 2A and FIG. 2B illustrate details of a semiconductor laser inaccordance with the first embodiment;

FIG. 3A through FIG. 3D illustrate segments in a waveguide core;

FIG. 4A and FIG. 45 illustrate a connection between a heater and anelectrode;

FIG. 5 illustrates details of a semiconductor laser in accordance with asecond embodiment;

FIG. 6 illustrates details of a semiconductor laser in accordance with athird embodiment; and

FIG. 7A through FIG. 7G illustrate measurement results of reflectionpeaks of a CSG-DR region of each semiconductor laser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given of embodiments of the present inventionwith reference to the accompanying drawings.

First Embodiment

FIG. 1 illustrates a laser device 100 in accordance with a firstembodiment of the present invention. As shown in FIG. 1, the laserdevice 100 has a semiconductor laser 200 and a controller 300. Thecontroller 300 has a central processing unit (CPU), a random accessmemory (RAM), a read only memory (ROM) and so on. The ROM of thecontroller 300 stores control information, a control program and so onof the semiconductor laser 200. The controller 300 controls a laseroscillation wavelength of the semiconductor laser 200 with an electricalsignal such as a current provided to the semiconductor laser 200.

FIG. 2A and FIG. 22 illustrate details of the semiconductor laser 200.FIG. 2A illustrates a top view of the semiconductor laser 200. FIG. 28illustrates a cross sectional view taken along a line A-A of FIG. 2A. Adescription will be given, with reference to FIG. 2A and FIG. 2B, of thesemiconductor laser 200. As shown in FIG. 2A and FIG. 2B, thesemiconductor laser 200 has a structure in which a Chirped SampledGrating Distributed Reflector (CSG-DR) region A and a Sampled GratingDistributed Feedback Laser (SG-DFB) region B are coupled in order.

The CSG-DR region A has a structure in which a waveguide core 3, acladding layer 6 and an insulating layer 8 are laminated on a substrate1 in order and heaters 11 a through 11 d, electrodes 12 and a groundelectrode 13 are laminated on the insulating layer 8. The SG-DFB regionB has a structure in which a waveguide core 4, the cladding layer 6, acontact layer 7 and an electrode 14 are laminated on the substrate 1 inorder.

The substrate 1 and the cladding layer 6 of the CSG-DR region A and theSG-DFB region B are a single layer formed as a unit respectively. Thewaveguide cores 3 and 4 are formed on a same plane and form a singlewaveguide core together. A low reflecting coating 9 is formed on endfaces of the substrate 1, the waveguide core 4 and the cladding layer 6at the SG-DFB region B side. On the other hand, a low reflecting coating10 is formed on end faces of the substrate 1, the waveguide core 3 andthe cladding layer 6 at the CSG-DR region A side. Diffractive gratings 2are formed at a given interval in the waveguide cores 3 and 4. Thesampled grating is thus formed.

The substrate 1 is, for example, a semiconductor substrate composed ofInP. The waveguide core 3 is, for example, composed of InGaAsPcrystalline having an absorption edge wavelength at shorter wavelengthsside compared to the laser oscillation wavelength. PL wavelength of thewaveguide core 3 is approximately 1.3 μm. The waveguide core 4 is, forexample, an active layer composed of InGaAsP crystalline amplifying alight of a desirable wavelength of a laser oscillation. The PLwavelength of the waveguide core 4 is approximately 1.55 μm.

Coupling constant of the diffractive grating 2 is approximately 200cm⁻¹. Pitch of the diffractive grating 2 is approximately 0.24 μm. Thenumber of asperity of the diffractive grating 2 is approximately 17.Thus, length of the diffractive grating 2 is approximately 4 μm. Braggwavelength of the diffractive grating 2 is approximately 1.55 μm. Inthis case, the reflectivity against the bragg wavelength of thediffractive grating 2 is approximately 1%.

Four segments are formed in the waveguide core 3 in the embodiment.Here, the segment is a region in which one region having the diffractivegrating 2 and one space region not having the diffractive grating 2 arecombined in the waveguide core. In the embodiment, the diffractivegrating 2 is formed at both ends of the waveguide core 3. And threediffractive gratings 2 are formed at a given interval between the twodiffractive gratings 2. In this case, it is defined that four segmentsincluding the diffractive grating 2 at the end on the side of the lowreflecting coating 10 are provided. And it is defined that four segmentsincluding the diffractive grating 2 at the end on the side of the SG-DFBregion B are provided. Details of each length of the segment in thewaveguide core 3 are given later.

It is preferable that the number of the segments of the waveguide core 4is the same as that of the waveguide core 3. For example, the number ofthe segments of the waveguide core 4 is four. Each optical length of thesegments in the waveguide core 4 is substantially equal to each other,and approximately 180 μm for example. “Substantially equal” means thatdifferences between each length of the space regions are less than 1% ofthe average length of the space regions.

The cladding layer 6 is composed of InP, constricts a current andconfines a laser light traveling in the waveguide cores 3 and 4. Thecontact layer 7 is composed of InGaAsP crystalline. The insulating layer8 is a protection film composed of an insulator such as SiN or SiO₂. Thelow reflecting coatings 9 and 10 are, for example, composed of adielectric film including MgF₂ and TiON. The reflectivity of the lowreflecting coatings 9 and 10 is, for example, less than 0.3%.

The heaters 11 a through 11 d are composed of such as NiCr, and arearranged from the low reflecting coating 10 toward the SG-DFB region Bon the insulating layer 8 above each of the segments in the waveguidecore 3. The heaters 11 a through 11 d control the temperature of thesegments in the waveguide core 3 according to the current intensityprovided from the controller 300 in FIG. 1. Each of the heaters 11 athrough 11 d is connected to each of the electrodes 12. The groundelectrode 13 is connected to the heaters 11 a through 11 d. Theelectrodes 12, the ground electrode 13 and the electrode 14 are composedof a conductive material such as Au.

FIG. 3A through FIG. 3D illustrate each of the segments in the waveguidecore 3. FIG. 3A illustrates a schematic view of the waveguide core 3. Asshown in FIG. 3A, a segment 31, a segment 32 and two segments 33 arecombined in order from the low reflecting coating 10 side to the SG-DFBregion B side. In FIG. 3A, the segment 33 on the side of the SG-DFBregion B is not shown. The length of the segment 31 is referred to asL1. The length of the segment 32 is referred to as L2. The length of thesegment 33 is referred to as L3.

L1 and L2 are different from each other. This results in that at leasttwo segments in the waveguide core 3 have a different length from eachother. In this case, there is a wavelength dependence of peak reflectionintensity of a light from the SG-DFB region B into the CSG-DR region A,in the waveguide core 3. That is, the peak reflection intensity isrelatively high in a given wavelength range. It is possible to restraina laser oscillation at a wavelength other than the desirable wavelengthwhen a lasing wavelength is one of wavelengths in the wavelength rangewhere the peak reflection intensity is relatively high.

However, there may be a laser oscillation at another wavelength becauseof regression wavelength caused by the vernier effect. And so, in theembodiment, the segment 33 has a length that is an average between thelength of the segment 31 and that of the segment 32. That is, L3 is anaverage between L1 and L2. The length of each segment may be adjustedwith a length of the space region.

FIG. 3B illustrates a reflection spectrum of an optical waveguide thatis formed with the segments 31 and 32. FIG. 3C illustrates a reflectionspectrum of an optical waveguide that is formed with the segments 31 and33 or the segments 32 and 33. FIG. 3D illustrates a reflection spectrumof the waveguide core 3. In FIG. 3B through FIG. 3D, a vertical axisshows a reflectivity, and a horizontal axis shows a wavelength. Inaddition, FIG. 3B through FIG. 3D illustrate an envelope curveconnecting each top of the reflection peak.

As shown in FIG. 3B, there is an envelope curve peak at a givenwavelength interval in the optical waveguide formed with the segment 31and the segment 32. Here, the envelope curve peak means a peak area inthe envelope curve. An interval between each envelope curve peak isrelatively reduced, because a difference between L1 and L2 is relativelylarge. In this case, a peak width at each envelope curve peak isreduced. This results in reduction of the number of the reflection peakhaving high intensity in each envelope curve peak. And there isincreased a difference between each peak reflection intensity in oneenvelope curve peak. It is therefore possible to restrain a laseroscillation at another wavelength in each envelope curve peak. However,the interval between each envelope curve peak is reduced. That is, thereis reduced a wavelength interval between each reflection peak having adesirable reflection peak intensity. Therefore, there may be a laseroscillation at another wavelength caused by the regression wavelength.

On the other hand, as shown in FIG. 3C, there is an envelope curve peakat a wavelength interval that is twice in a case of FIG. 3B, in theoptical waveguide formed with the segments 31 and 33 or the segments 32and 33. This is because the length difference is reduced to half,compared to a case of FIG. 3B. In this case, the interval of eachenvelope curve peak is increased. There is increased the wavelengthinterval of each reflection peak having the desirable reflection peak.It is therefore possible to restrain the laser oscillation at anotherwavelength caused by the regression wavelength. However, the peak widthof each envelope curve peak is increased. Therefore, each envelope curvepeak includes a plurality of reflection peaks having relatively highintensity. In this case, one of the envelope curve peaks may include aplurality of reflection peaks having the desirable reflection peakintensity. Therefore, there may be a laser oscillation at anotherwavelength other than the desirable wavelength.

And so, in the embodiment, the optical waveguide is formed with thesegments 31 and 32 and two of the segments 33. In this case, as shown inFIG. 3D, the envelope curve peaks in the optical waveguide formed withthe segments 31 and 32 are alternately reduced because of antiphasepeaks of the reflection peak intensity of the optical waveguide formedwith the segments 31 and 33 or the segments 32 and 33. That is, theintensity of the reflection peaks having the desirable reflection peakintensity is alternately reduced. This results in increase of thewavelength interval of the reflection peak having the desirablereflection peak. It is therefore possible to restrain the laseroscillation at a wavelength other than the desirable wavelength. In theembodiment, a plurality of the segments 33 are formed. This results inincrease of width reduction of the reflection peak caused by theantiphase peak. It is therefore possible to further restrain the laseroscillation at a wavelength other than the desirable wavelength.

Consequently, it is possible to enlarge the wavelength interval of thereflection peak having the desirable reflection peak intensity, if theoptical waveguide is formed with the segments 31 and 32 and the twosegments 33 and the number of the reflection peaks having relativelyhigh intensity in each of envelope curve peak. It is therefore possibleto restrain the laser oscillation at a wavelength other than thedesirable wavelength.

It is preferable that the segment having the length L3 is positionedcloser to the gain region than the segments having the lengths L1 andL2. This is because the reflection peak intensity is further reduced inboth of wavelength ranges adjacent to the reflection peak having thedesirable reflection peak intensity. It is therefore preferable that thesegments 33 are positioned closer to the SG-DFB region B than thesegments 31 and 32, in the embodiment.

There may be three segments having the length L3. In this case, theantiphase peak intensity gets higher. It is therefore possible to reducethe reflection peak intensity in both of the wavelength ranges adjacentto the reflection peak having the desirable reflection peak intensity.There may be more than two segments having the length L1 or more thantwo segments having the length L2. In this case, it is possible torestrain the laser oscillation at a wavelength other than the desirablewavelength. It is however preferable that N3≧(N1+N2)/2 when the numberof the segment having the length L1 is N1, the number of the segmenthaving the length L2 is N2 and the number of the segment having thelength L3 is N3. This is because the reflection peak intensity in bothof the wavelength ranges adjacent to the reflection peak having thedesirable reflection peak intensity may be reduced further.

In addition, it is preferable that the wavelength interval between eachreflection peak having the desirable reflection peak intensity is morethan 50 nm. This is because the laser oscillations at the adjacentreflection peak are restrained. It is preferable that the interval ofthe longitudinal mode is approximately 1 nm to 2 nm, in view ofcontrolling of the equivalent refractive index. It is thereforepreferable that (|L1−L3|/L3)=(|L3−L2|/L3)=|ΔL3| is approximately 1/50(2%) to 2/50 (4%).

The length of the waveguide core 3 changes according to the changing ofthe equivalent refractive index of each segment. Here, the equivalentrefractive index of the segment 31 is n1, the equivalent refractiveindex of the segment 32 is n2, the equivalent refractive index of thesegment 33 is n3. The envelope curve peaks in the optical waveguideformed with the segments 31 and 32 are alternately overlapped with theenvelope curve peak of the optical waveguide formed with the segments 31and 33 or the segments 32 and 33, when each equivalent refractive indexof the segments changes along the following equation shown as n3(n1+n2)/2. Therefore, the reflection peak intensity of the opticalwaveguide formed with the segments 31 and 32 are alternately reducedbecause of antiphase peaks of the reflection peak intensity of theoptical waveguide formed with the segments 31 and 33 or the segments 32and 33. It is therefore possible to change the lasing wavelength, withthe laser oscillation at a wavelength other than the desirablewavelength being restrained.

Next, a description will be given of an operation of the laser device100. At first, a given current is provided to the electrode 14 from thecontroller 300 in FIG. 1. And a light is generated in the waveguide core4. The light propagates in the waveguide cores 3 and 4, is reflected andamplified repeatedly, and is emitted toward outside. In this case, it ispossible to restrain a lasing at other than the desired wavelength, asshown in FIG. 3.

The controller 300 controls the equivalent refractive index of thesegments in the waveguide core 3, when the controller 300 controls thelasing wavelength. In the embodiment, the controller 300 controls therefractive index of the segments by controlling the temperature of thesegments. Here, the temperature of the segment 31 is referred to as T1.The temperature of the segment 32 is referred to as T2. The temperatureof the segment 33 is referred to as T3. The controller 300 may controlthe temperature with electrical power provided to the heaters 11 athrough 11 d.

Each wavelength of the reflection peak is shifted with magnituderelation between, each reflection peak intensity in the CSG-DR region Abeing maintained, when the temperature T1 and the temperature T2 changewith the temperature T1 being different from the temperature T2. Themagnitude relation between each reflection peak changes with thewavelength of each reflection peak being kept constant, when adifference between the temperature T1 and the temperature T2 (atemperature gradient in the CSG-DR region A) changes. It is possible toobtain the desirable wavelength by controlling the temperature T1, thetemperature T2 and the difference between the temperature T1 and thetemperature T2.

In the embodiment, the temperature T3 of the segment 33 is kept to be anaverage between the temperature T1 and the temperature T2(T3=(T1+T2)/2), even if the desirable wavelength is obtained with thecontrol. This results in increase of the wavelength interval of thereflection peak having relatively high intensity. The controller 300realizes the temperatures T1 through T3 having the above-mentionedrelationship at every desirable wavelength with respect to thesemiconductor laser 200.

It is necessary to prepare control data (electrical potential of theheaters in a case of temperature control) for controlling refractiveindex of the segments at every wavelength, in order to control eachsegment with the controller 300. For example, the number of the controldata is 264 when there are three heaters for controlling temperature andthere are 88 channels for selecting wavelength. In this case, thecontroller 300 can control wavelength if the controller 300 stores thecontrol data.

However, it is necessary to generate the data by measuring outputwavelength with a wavelength meter and recording the electricalpotential of an objective heater before shipment of the laser.Therefore, it is a burden that there are many control data, when thecontrol data are generated. Most of hardware resource such as memory isconsumed if the controller 300 stores and keeps many control data andcontrols with the control data.

There may be a method of complementing a value of refractive index withan average, in order to solve the problem. Specifically, it is possibleto determine an intermediate temperature of a heater by storing amaximum temperature and a minimum temperature of a heater temperature asthe control data and generating an average between the two temperaturesin a case where three different temperatures are needed, because eachrefractive index of the segments is controlled with a constantlinearity.

With the method, the 264 control data may be reduced to 176 controldata. This results in simplification of the generation of the controldata and reduction of hardware resource during the use of the laser.That is, the control data may be simplified, if the controller 300stores the control data of the temperature T1 of the segment 31 and thetemperature T2 of the segment 32 and the controller 300 generates thecontrol data of the temperature T3 of the segment 33 that is shown asT3−(T1+T2)/2. This method may be applied to a device not having thesegment 33.

In the embodiment, the heaters 11 c and 11 d are controlled so that eachof the segments 33 has the same temperature. In this case, a commonelectrical potential may be used for controlling the heaters 11 c and 11d. Therefore, the number of the temperature parameters is three althoughthere are four segments of which temperature is controllable. It istherefore possible to control the oscillation wavelength easily.

As shown in FIG. 4A, the heaters 11 c and 11 d may be connected to acommon electrode 12 a. In this case, it is preferable that the segmentshaving the length L3 are adjacent to each other, because it is easy toarrange the common electrode 12 a. It is preferable that the segmentshaving the length L3 are adjacent to each other, if the number of thesegments having the length L3 is more than three. In addition, as shownin FIG. 4B, the heaters 11 c and 11 d may be connected to a commonelectrode outside of the semiconductor laser 200.

When the number of the segment having the length L1 is more than one, itis preferable that the segments are adjacent to each other, because itis possible to use a common electrical potential for controlling thetemperature of the segments. It is preferable that the segments havingthe length L2 are adjacent to each other, if the number of the segmentshaving the length L2 is more than one. Therefore, the number of thetemperature parameter is three, even if the number of the segment isincreased. It is therefore possible to control the oscillationwavelength.

In the embodiment, the segment 31 corresponds to the first segment, thesegment 32 corresponds to the second segment, the segment 33 correspondsto the third segment, the heaters 11 a through 11 d correspond to therefractive index control portion, the SG-DFB region B corresponds to thegain region, and the semiconductor laser corresponds to the opticalsemiconductor device.

In addition, K1 may be changed in a range of 0.3≦K1≦0.7 as necessary,although L3 is an average of L1 and L2 as shown L3=(L1+L2)×K1 (=0.5). Inthis range, it is possible to reduce the number of the reflection peakhaving the relatively high intensity in each of the envelope curvepeaks. And it is possible to enlarge the wavelength interval of thereflection peak having the desirable reflection peak. In a case wherethe equivalent refractive indexes n1 and n2 are controlled so as todiffer from each other with temperature control or the like (n1≠n2),each refractive index is controlled along the equation shown asn3=n1+(n2−n1)×K2 (0.3≦K2≦0.7). This results in increase of thewavelength interval of the reflection peak having the desirablewavelength peak intensity. For example, each temperature is controlledalong the equation shown as T3=T1+(T2−T1)×K3 (0.3≦K3≦0.7), when theequivalent refractive index is controlled with temperature control. T1or T2 may be calculated, although T3 is calculated with theabove-mentioned equation.

Second Embodiment

Next, a description will be given of a semiconductor laser 200 a inaccordance with a second embodiment of the present invention. FIG. 5illustrates a schematic cross sectional view of the semiconductor laser200 a. As shown in FIG. 5, the semiconductor laser 200 a has an opticalpower control (PC) region C, being different from the semiconductorlaser 200 shown in FIG. 1.

The PC region C has a structure in which a waveguide core 5, thecladding layer 6, a contact layer 15 and an electrode 16 are laminatedon the substrate 1 in order. In the embodiment, the low reflectingcoating 9 is provided on end faces of the substrate 1, the waveguidecore 5 and the cladding layer 6 at the PC region C side.

The substrate 1 and the cladding layer 6 of the CSG-DR region A, theSG-DFB region B and the PC region C are a single layer formed as a unitrespectively. The waveguide cores 3, 4 and 5 are formed on a same planeand form a single waveguide core together. The insulating layer 8 isfurther formed so as to insulate the contact layer 15 and the electrode16 from the contact layer 7 and the electrode 14. The waveguide core 5may have the same structure as the waveguide core 3 or 4. An opticaloutput changes according to an increase of loss caused by currentinjection, when the waveguide core 5 has the same structure as thewaveguide core 3. The optical output changes according to a generationof gain caused by the current injection, when the waveguide core 5 hasthe same structure as the waveguide core 4. The electrodes 14 and 16 arecomposed of a conductive material such as Au. The contact layer 15 iscomposed of InGaAsP crystalline.

Next, a description will be given of an operation of the semiconductorlaser 200 a. At first, a given current is provided to the electrode 14from the controller 300 in FIG. 1. And a light is generated in thewaveguide core 4. The light propagates in the waveguide cores 3, 4 and5, is reflected and amplified repeatedly, and is emitted toward outside.A given current is provided to the electrode 16 from the controller 300.And an output of the emitted light is kept constant. In this case, it ispossible to restrain a lasing at a wavelength other than the desirablewavelength, as shown in FIG. 3.

In the embodiment, the PC region C corresponds to the light absorptionregion or a light amplification region.

Third Embodiment

Next, a description will be given of a semiconductor laser 200 b inaccordance with a third embodiment of the present invention. FIG. 6illustrates a schematic cross sectional view of the semiconductor laser200 b. As shown in FIG. 6, the semiconductor laser 200 a has a SampledGrating Distributed Reflector (SG-DR) region D instead of the SG-DFBRegion B, and further has a Gain region E and a Phase Shift (PS) regionF. In the semiconductor laser 200 b, the Gain region E and the PS regionF are combined to each other between the CSG-DR region A and the SG-DRregion D.

The SG-DR region D has a structure in which a waveguide core 17, thecladding layer 6 and the insulating layer 8 are laminated in order onthe substrate 1. The Gain region E has a structure in which a waveguidecore 18, the cladding layer 6, a contact layer 19 and an electrode 20are laminated in order on the substrate 1. The PS region F has astructure in which a waveguide core 21, the cladding layer 6, a contactlayer 22 and an electrode 23 are laminated in order on the substrate 1.

The substrate 1 and the cladding layer 6 of the CSG-DR region. A, theSG-DR region D, the Gain region E and the PS region F are a single layerformed as a unit respectively. The waveguide cores 3, 17, 18 and 21 areformed on a same plane and form a single waveguide core together. Theinsulating layer 8 is further formed so as to insulate the electrode 20and the contact layer 19 from the electrode 23 and the contact layer 22.

The waveguide core 17 is, for example, composed of InGaAsP crystallinehaving an absorption edge wavelength at shorter wavelengths sidecompared to the laser oscillation wavelength. The diffractive gratings 2are formed at a given interval in the waveguide cores 17. The sampledgrating is thus formed. Each optical length of the segments in thewaveguide core 17 is substantially equal to each other. “Substantiallyequal” means that differences between each length of the space regionsare less than 1% of the average length of the space regions.

The waveguide core 18 is, for example, composed of InGaAsP crystallinehaving a gain with respect to the laser oscillation at the desirablewavelength. The PL wavelength of the waveguide core 18 is approximately1.55 μm. The waveguide core 21 is, for example, composed of InGaAsPcrystalline having an absorption edge wavelength at shorter wavelengthsside compared to the laser oscillation wavelength. The contact layers 19and 22 are composed of InGaAsP crystalline. The electrodes 20 and 23 arecomposed of a conductive material such as Au.

Next, a description will be given of an operation of the semiconductorlaser 200 b. At first, a given current is provided to the electrode 20from the controller 300 in FIG. 1. And a light is generated in thewaveguide core 18. The light propagates in the waveguide cores 3, 17, 18and 21, is reflected and amplified repeatedly, and is emitted towardoutside. A given current is provided to the electrode 23 from thecontroller 300. And a phase of the propagating light is controlled inthe waveguide core 21. In this case, it is possible to restrain a lasingat other than the desired wavelength, as shown in FIG. 3. In addition,the semiconductor laser 200 b may further have another CSG-DR region Ainstead of the SG-DR region D.

EXAMPLES

Characteristics of the semiconductor laser in accordance with theabove-mentioned embodiment were measured with computer simulation.

First Example

The semiconductor laser 200 in accordance with the first embodimentshown in FIG. 2 was manufactured in a first example. In the firstexample, the length L1 was 196 μm, the length L2 was 208 μm, and thelength L3 was 202 μm.

Second Example Through Fourth Example

Semiconductor lasers, of which number of each segment in the waveguidecore 3 is different from that of the first example, were manufactured ina second example through a fourth example. The other structure was thesame as that of the first example. Table 1 shows the number of eachsegment of each example. For example, the number of the segment havingthe length L1 through L3 is respectively two in the second example.

TABLE 1 L1 L2 L3 FIRST EXAMPLE 1 1 2 SECOND EXAMPLE 2 2 2 THIRD EXAMPLE2 2 3 FOURTH EXAMPLE 3 2 3 FIRST COMPARATIVE EXAMPLE 1 1 0 SECONDCOMPARATIVE EXAMPLE 1 1 1

The segments having the length L3 are continuously arranged on theSG-DFB region B side. The segments having the length L2 are continuouslyarranged on the low reflecting coating 10 side. The segments having thelength L1 are continuously arranged between the segments having thelength L2 and the segments having the length L3.

Fifth Example

A semiconductor laser, in which two segments 32, three segments 33 andtwo segments 31 are arranged in order from the SG-DFB region side, wasmanufactured in a fifth example. The other structure was the same asthat of the third example.

First Comparative Example

A semiconductor laser not having the segment 33 was manufactured. Theother structure was the same as that of the first example.

Second Comparative Example

A semiconductor laser, of which number of the segments 31 through 33 isrespectively one, was manufactured. The other structure was the same asthat of the first example.

[Analysis]

The reflection spectrum was measured with respect to the CSG-DR regionof each semiconductor laser. FIG. 7A through FIG. 7G illustrate ameasurement result. In FIG. 7A through FIG. 7G, a vertical axis shows areflectivity, and a horizontal axis shows a wavelength. A curve line ineach figure shows an envelope curve connecting each top of reflectionpeak. Numbers in parenthesis in each figure show those of the segments,and show the number of the segments having the length L1 through L3 in aleft to right.

As shown in FIG. 7A, a wavelength interval of the envelope curve wasrelatively small in the semiconductor laser of the first comparativeexample. Therefore, there may be a laser oscillation at a wavelengthother than a desirable wavelength in the semiconductor laser of thefirst comparative example. As shown in FIG. 7B, the envelope curve peakswere alternately reduced in the second comparative example. However, areflectivity difference was relatively small between the peak havingrelatively high reflectivity and the peak having relatively lowreflectivity. Therefore, there may be a laser oscillation at awavelength other than a desirable wavelength in the semiconductor laserof the second comparative example.

On the other hand, as shown in FIG. 7C, the envelope curve peaks of thesegment 31 and the segment 32 were alternately lowered because of thetwo segments 33, in the semiconductor laser of the first example.Therefore, there was increased the wavelength interval of eachreflection peak having the desirable wavelength. And there was increasedthe reduction width of the envelope curve peak of which reflectivity waslowered as shown in FIG. 7B. Consequently, it is possible to restrainthe laser oscillation at a wavelength other than the desirablewavelength, in the semiconductor laser of the first example.

As shown in FIG. 7D through FIG. 7F, there was increased the wavelengthinterval of each reflection peak having the desirable reflection peak,because the semiconductor lasers of the second example through thefourth example had a plurality of the segments having the length L3. Itis therefore possible to restrain the laser oscillation at a wavelengthother than the desirable wavelength.

As shown in FIG. 7G, there was reduced the reduction width of theenvelope curve peak of which reflectivity was lowered as shown in FIG.7B, in the semiconductor laser of the fifth example. This results inthat the segment 33 is preferably positioned on the SG-DFB region B sidecompared to the segments 31 and 32.

The present invention is not limited to the specifically disclosedembodiments, but include other embodiments and variations withoutdeparting from the scope of the present invention.

The present application is based on Japanese Patent Application Nos.2007-099921 filed on Apr. 5, 2007 and 2008-050923 filed on Feb. 29,2008, the entire disclosure of which is hereby incorporated byreference.

1. An optical semiconductor device comprising: a waveguide that has oneor more first segments, one or more second segments and a plurality ofthird segments, the first segment having a region that includes adiffractive grating and another region that is a space region combinedto the region, the second segment having a region that includes adiffractive grating and another region that is a space region combinedto the region, a length of the second segment being different from thatof the first segment, the third segment having a region that includes adiffractive grating and another region that is a space region combinedto the region, a length of the third segment being shown asL3=L1+(L2−L1)×K1 in which 0.3≦K1≦0.7, L1 is a length of the firstsegment, L2 is a length of the second segment and L3 is a length of thethird segment; and a refractive index control portion that controlsrefractive index of the first segment through the third segments.
 2. Theoptical semiconductor device as claimed in claim 1, wherein anequivalent refractive index of the third segment is an average betweenthe equivalent refractive indexes of the first and the second segments.3. The optical semiconductor device as claimed in claim 1, wherein thesame type segments are adjacent to each other if the number of the sametype segment is more than one.
 4. The optical semiconductor device asclaimed in claim 1, wherein the refractive index control portion is aplurality of heaters.
 5. The optical semiconductor device as claimed inclaim 4 further comprising a plurality of heaters according to aplurality of the same type segments, wherein each electrode of theheaters is connected to a common electrode.
 6. The optical semiconductordevice as claimed in claim 1 further comprising a gain region that isoptically connected to the first through the third segments.
 7. Theoptical semiconductor device as claimed in claim 6, wherein the thirdsegment is closer to the gain region than the first segment and thesecond segment.
 8. The optical semiconductor device as claimed in claim1 further comprising a plurality of diffractive grating regions thathave a region including a diffractive grating and another region being aspace region combined to the region and are optically connected to thefirst through the third segments, each length of the diffractive gratingregions being substantially equal to each other.
 9. The opticalsemiconductor device as claimed in claim 1 further comprising at leastone of a light absorption region and a light amplification region thatare optically connected to the first through the third segments.
 10. Theoptical semiconductor device as claimed in claim 1, wherein the numberof the third segment is equal to or more than an average of the numberof the first segment and the second segment.
 11. The opticalsemiconductor device as claimed in claim 1, wherein a length differencebetween the first segment and the second segment is between 2% to 4% ofthe length of the third segment.
 12. The optical semiconductor device asclaimed in claim 1, wherein the length of the third segment is anaverage of the length of the first segment and the length of the secondsegment.
 13. A method of controlling an optical semiconductor device,comprising controlling refractive index of a first segment through athird segment by controlling a refractive index control portion with useof a following expression shown as n3=n1+(n2−n1)×K2 in which 0.3≦K2≦0.7,n1 is an equivalent refractive index of the first segment, n2 is anequivalent refractive index of the second segment and n3 is anequivalent refractive index of the third segment, the opticalsemiconductor device having the refractive index control portion and awaveguide including the first through the third segment, the firstsegment having a region that includes a diffractive grating and anotherregion that is a space region combined to the region, the second segmenthaving a region that includes a diffractive grating and another regionthat is a space region combined to the region, a length of the secondsegment being different from that of the first segment, the thirdsegment having a region that includes a diffractive grating and anotherregion that is a space region combined to the region, a length of thethird segment being shown as L3=L1+(L2−L1)×K1 in which 0.3≦K1≦0.7, L1 isa length of the first segment, L2 is a length of the second segment andL3 is a length of the third segment, the refractive index controlportion controlling the refractive index of the first through the thirdsegments.
 14. The method as claimed in claim 13, wherein the equivalentrefractive index is controlled with temperature of the first through thethird segments.
 15. The method as claimed in claim 13, wherein theequivalent refractive index of the third segment is an average of theequivalent refractive index of the first segment and the equivalentrefractive index of the second segment.
 16. A method of controlling anoptical semiconductor device comprising: getting two of control pointsaccording to each of a first segment through a third segment;calculating a lacking control point with use of a following expressionshown as T3=T1+(T2−T1)×K3 in which 0.3≦K3≦0.7, T1 is a control point ofthe first segment, T2 is a control point of the second segment and T3 isa control point of the third segment; and controlling each refractiveindex of the first segment through the third segment with the controlpoints according to each of the segment, the optical semiconductordevice having a waveguide including the first through the third segment,the first segment having a region that includes a diffractive gratingand another region that is a space region combined to the region, thesecond segment having a region that includes a diffractive grating andanother region that is a space region combined to the region, a lengthof the second segment being different from that of the first segment,the third segment having a region that includes a diffractive gratingand another region that is a space region combined to the region, alength of the third segment being shown as L3=L1+(L2−L1)×K1 in which0.3≦K1≦0.7, L1 is a length of the first segment, L2 is a length of thesecond segment and L3 is a length of the third segment.
 17. The methodas claimed in claim 16, wherein the first through third segments are aplurality of segments having different length from each other andanother segment having an average length of the plurality of thesegments.
 18. The method as claimed in claim 17, wherein the number ofthe segment having the average length is more than one.
 19. The methodas claimed in claim 16 further comprising complementing the lackingcontrol point with use of an average of the control points of the firstsegment and the second segment.